Copper chromite-alumina catalysts having high-temperature stability

ABSTRACT

The activity and stability of copper chromite catalysts have been found to be sensitive to both copper-to-chromium ratio and to alumina content. Alumina-containing catalysts of improved stability for the oxidation of carbon monoxide at high temperatures and the use of these catalysts in the control of automotive exhaust emissions are described.

Farrauto et al.

COPPER CHROMITE-ALUMINA CATALYSTS HAVING HIGH-TEMPERATURE STABILITY Inventors: Robert J. Farrauto, Painted Post;

Karl E. Hoekstra, Corning; Robert D. Shoup, Painted Post, all of NY.

Assignee: Corning Glass Works, Corning, N.Y. Filed: Mar. 2, 1973 Appl. No.: 337,293

US. Cl. 252/465, 423/213.2 Int. Cl B0lj 11/22 Field of Search 252/465; 423/213.2

References Cited UNITED STATES PATENTS 4/1942 Burk et al. 252/465 X 9/1961 Moy et a1 252/465 X 12/1966 Kearby 252/465 X Mar. 11, 1975 3,410,651 11/1968 Brandenburg et a1. 252/465 X 3,444,099 5/1969 Taylor et al. 252/465 3,787,322 l/1974 Koberstein et al 252/465 FOREIGN PATENTS OR APPLICATIONS 1,138,444 1/1969 Great Britain 252/465 1,168,075 10/1969 Great Britain Primary Examiner-Winston A. Douglas Assistant ExaminerW. J. Shine Attorney, Agent, or Firm-Clarence R. Patty, .lr.; Kees van der Sterre [57] ABSTRACT 2 Claims, 3 Drawing Figures HAR I 1 I975 DATENI COPPER CHROMITE-ALUMINA CATALYSTS HAVING HIGH-TEMPERATURE STABILITY BACKGROUND OF THE INVENTION Copper-chromium-containing catalysts have a variety of applications in industrial processing for hydrogenation and dehydrogenation reactions as well as certain selective oxidation processes. The temperatures required for these processes, however, typically do not exceed 500C. and thus thermal stability in the catalyst is not a major problem, at least initially.

More recently, copper chromite catalysts have become primary candidates for automotive emissions control. Proposed methods to control auto emissions via catalytic devices involve inserting a dispersed catalyst supported by inert beads or a monolithic honeycomb structure in the automobile exhaust train. Starting at temperatures of about 200F and above in the presence of excess oxygen, the catalyst is expected to convert major proportions of noxious emissions such as carbon monoxide and unburned hydrocarbons to harmless carbon dioxide and water in accordance with government specifications. However, for automotive exhaust applications the catalyst must be able to control CO and hydrocarbon emissions in an environment wherein sustained temperatures in excess of 300C. and occasional excursions to 800C. or higher are encountered.

Copper chromite catalysts comprising the phases cupric chromite (CuCr- O,) and, optionally, cupric oxide (CuO) are quite active for the oxidation of carbon monoxide and hydrocarbons, but are not inherently stable at temperatures in the 800C. range. This is evidenced by the fact that sustained heating of copper chromites at such temperatures produces substantial reductions in activity for the oxidation of hydrocarbons and carbon monoxide. These heat-induced reductions in activity are presently attributed to the growth of a cuprous chromite (Cu Cr O,) phase which exhibits larger average particle size than CuCr O.,.

It was thought that the stabilization of copper chromite catalysts against the formation of Cu Cr O at high temperatures would improve their usefulness for automotive exhaust applications. The formation of Cu C- r 0, at temperatures below about 900C. occurs through the slow reaction: CuO-I-CuCrZO, Cu C- r O,+ Thus a limited degree of success in improving the thermal stability of copper chromite catalysts has in the past been achieved through the use of metal oxide additives to suppress Cu Cr- O, formation and/or through the removal of the reactive copper oxide phase from the catalyst prior to use, for example, by leaching.

Alumina is also widely used in the field of catalysis, both as an active catalytic agent and as an inert support material for other catalysts. For example, U.S. Pat. No. 2,280,060 describes coke-inhibiting Al O -CuO-Cr- 0 catalysts useful in the aromatization of hydrocarbons. Alumina is also quite commonly employed as a support material for other catalysts in hightemperature oxidative applications such as automotive exhaust emissions control systems because it combines the desirable properties of inertness high surface area and refractoriness. Alumina-supported noble metal catalysts, for example, provide a very high degree of activity for the oxidation of carbon monoxide and hydrocarbons under high-temperature conditions and are very strong candidates for automotive emissions con- SUMMARY OF THE INVENTION Our invention is based on the discovery that certain active copper chromite catalysts can be thermally stabilized through the addition of specified quantities of alumina thereto. We have found that the suppression of Cu Cr O, formation can be realized by limiting the excess CuO phase through proper stoichiometric control of CuO, Cr O and A1 0 with leaching being employed as an additional tool to remove excess CuO, if desired.

Thus in one aspect our invention comprises leached copper chromite-alumina catalysts of improved stability having an alumina content ranging about 5-25% by weight. These catalysts have a copper-to-chromium atom ratio of about 0.511 as the result of leaching to remove free CuO, and consist essentially ofonly CuCr O and A1 0 crystal phases with at most only minor amounts of CuCr O Al O solid solution. The catalysts resist the formation of Cu Cr O and demonstrate better thermal stability than alumina-free leached copper chromites.

In a second aspect, our invention comprises unleached copper chromite-alumina catalysts of improved stability having a copper-to-chromium atom ratio in the range of about 0.5-1.0, an aluminum-tochromium ratio R in the range of about 02-] .0, and a minimum copper content, calculated as CuO, of at least-about [45 5 (R 0.2)/0.8] mole percent of the composition, R being the aluminum-to-chromium ratio as set forth above. These catalysts contain CuO, CuCr- 0,, and CuAl O, crystal phases, are resistant to the formation of undesirable Cu Cr O and are active for the oxidation of carbon monoxide and hydrocarbons to CO and water.

The improved thermal stability of our copper chr"omite-alumina catalysts is evidenced by the retention of a high degree of activity for the oxidation of carbon monoxide and hydrocarbons after exposure to elevated temperatures, a property which renders these catalysts eminently suitable for use in oxidation processes at temperatures up to about 800C. Thus a further important aspect of the invention resides in the use of the described catalysts in processes such as the treatment of automotive exhaust gases which require the oxidation of these species at sustained elevated temperatures in the 600800C. range without excessive degradation.

DESCRIPTION OF THE DRAWING The dependence of thermal stability on the composition of catalysts in the copper chromite-alumina system can be seen in the drawing, wherein FIG. 1 is a graph showing the relationship between the alumina content of acid-leached copper chromitealumina catalysts containing essentially no free CuO and the resistance of the catalysts to degradation by accelerated thermal aging. Solid curve A reflects the carbon monoxide oxidation activity of catalysts after sintering at 600C. for 4 hours, while broken curve B and chained curve C show the changes in activity which occur after thermal aging for 24 hours in air at 800C. and IOC. respectively.

FIG. 2 of the DRAWING is a diagram of the copper oxide-chromium oxide-aluminum oxide ternary system wherein the region X encompasses unleached copper chromite-alumina catalysts within the scope of the present invention demonstrating improved stability at elevated temperatures.

FIG. 3 of the DRAWING is a subsolidus phase diagram of a portion of the copper oxide-chromium oxidealuminum oxide ternary system enlarged to show pertinent regions of this system for the purpose of comparing the stability of catalysts in these regions after accelerated thermal aging for 24 hours at 800C. in air.

DETAILED DESCRIPTION The improved thermal stability of the copper chromite-alumina catalysts of the present invention is not critically dependent upon the method used to prepare the catalyst. Any means which will insure thorough dispersion of alumina throughout the copper chromite phase in the proper proportions may be employed. One suitable method of preparation comprises providing a solution containing the nitrates of copper, aluminum, and chromium in proportions which will yield the desired composition in the prepared catalyst, drying the solution at 150C., and firing the dried product at 600C. for 24 hours to obtain a catalytically-active powder. Alternatively slurries containing finely-divided oxides such as colloidal alumina in combination with dissolved compounds of the other constituents may be dried and fired to product the catalyst either as an active coating on a refractory ceramic support or an unsupported powder.

As previously mentioned, it is desirable from the standpoint of thermal stability to provide copperchromite catalysts having a copper-to-chromium atom ratio of about 0.5: l wherein no excess CuO which may form cuprous chromite is present. This ratio can be attained by adjustment of the ratio of copper compounds to chromium compounds in the starting mixture; however. the preparation of highly active catalysts from starting mixtures wherein the copper-tochromium atom ratio approaches 0.5 (equivalent to stoichiometric CuCr O is difficult because incomplete reaction between Cu and Cr results in the presence of a relatively inactive Cr O phase. Thus in the preparation of copper chromite-alumina catalysts having copper-tochromium ratios near 0.5:l it is useful to prepare a starting mixture containing an excess of copper to insure complete reaction of all of the chromium, and thereafter to remove the excess CuO present in the prepared catalyst by leaching with a strong mineral acid. The removal of excess CuO by this means may be accomplished either before or after the addition of alumina to the catalyst, but the latter is preferred.

Leached copper chromite catalysts containing no excess CuO might be expected to demonstrate substantially better thermal stability than catalysts containing excess copper. Nevertheless we have discovered that even these copper chromite catalysts may be further stabilized against thermal degradation through the addition of controlled amounts of alumina. The stabilizing effect of alumina additions on leached copper chromite is readily apparent from the following example.

EXAMPLE I A series of samples of powdered crystalline copper chromite-alumina catalysts is prepared from aqueous slurries containing dissolved Cu(NO and Cr(NO together with measured additions of colloidal alumina. The slurries are first heated to 150C. to evaporate essentially all of the water, and thereafter fired at 600C. for 4 hours to convert the mixtures of dry nitrates to a series of copper chromite-alumina catalysts. The nitrates are present in the original solutions in proportions which will yield a copper-to-chromium atom ratio of about 1:1 in the tired materials, while the alumina content varies from sample to sample from a minimum of zero to a maximum of 100%. X-ray examination of these materials discloses the presence of A1 0 CuO, and CuCr O crystal phases. Excess CuO is then removed from the catalysts by treatment with boiling 6 M HCL for about an hour, followed by thorough water washing. The CuzCr ratio in the acid-leached catalysts is about 0.5:], and X-ray examination indicates that only A1 0 and CuCr O phases remain.

The powdered crystalline catalysts prepared as described are then tested for catalytic activity using a differential scanning calorimeter. The calorimeter permits the determination of the relative extent of oxidation of a flowing carbon monoxide-air mixture from the amount of heat generated in the course of the oxidation reaction. A comparison of activity between catalysts may then be made, based on the fact that more active catalysts achieve an equivalent degree of oxidation of carbon monoxide, e.g., 50% conversion to CO at lower reaction temperatures than do less active catalysts.

Curve A of FIG. 1 of the drawing is a plot reflecting the catalytic activity of a series of copper chromitebased catalysts prepared as described. Relative catalytic activities, compared on the basis of relative 50% carbon monoxide conversion temperatures (vertical axis), are plotted as a function of alumina content (horizontal axis). Alumina contents are in weight percent of the copper chromite-alumina samples and 50% conversion temperatures are in degrees Fahrenheit. It is evident from a study of curve A that catalysts containing alumina in amounts not exceeding about 35% by weight demonstrate a higher level of oxidation activity toward carbon monoxide than alumina-free copperchromite catalysts.

Curve B of FIG. 1 is a plot showing the effect on catalytic activity of accelerated thermal aging for 24 hours at 800C. in air for a similarly prepared series of leached copper chromite-alumina compositions. Curve B suggests that catalysts containing 5-25% alumina by weight retain a substantially greater proportion of their initial activity after exposure to this aging treatment than do catalysts in the 25-75% alumina range. Curve B also shows a second unexpected region of stability in the -85% alumina range; however the activity of these catalysts is substantially less and they are not deemed of practical utility for emissions control applications.

Curve C of FIG. I is a plot showing the effect on catalytic activity of further accelerated thermal aging for 24 hours at 1000C. in air for a similarly prepared series of leached copper chromite-alumina catalysts. This treatment causes some degradation even in the 5-25% alumina-containing catalysts; however, the substantially better stability of these compositions as compared with those containing increased additions of alumina is readily apparent.

The effect of alumina content on the thermal stability of copper chromite with respect to hydrocarbon oxidation is not as pronounced as in the case of carbon monoxide oxidation. Thus copper chromite-alumina catalysts which have been aged for 24 hours at 800C. dem onstrate a level of oxidation activity which is very nearly independent of alumina concentration, particularly in the 525% alumina region which is of interest for the oxidation of carbon monoxide. For this reason, thermal stability with respect to the oxidation of carbon monoxide is considered to be the most inportant factor to be controlled in optimizing composition in the copper chromite-alumina field. Leached copper chromitealumina catalysts which are particularly stable are those containing about 5l5% alumina by weight and essentially no excess CuO (a copper-to-chromium ratio of about 0.511

Notwithstanding the stabilizing effect of alumina on leached copper chromite catalysts as evidenced by the above data, it is in many cases desirable to obtain stabilization of the catalysts witout the need for leaching. Another important aspect of our invention therefore deals with the stabilization of unleached copper chromites containing alumina through the stoichiometric control of the copper-to-chromium and aluminum-tochromium atom ratios. In this way, catalysts characterized by the presence of CuO, CuCr O and CuAl O crystal phases which are resistant to the formation of Cu Cr O,, active for the complete oxidation of carbon monoxide and hydrocarbons, and of improved stability to temperatures of up to 800C. may be produced.

The preparation of an unleached catalyst of improved stability is described in detail in Example II below.

EXAMPLE ll A refractory honeycomb ceramic support structure composed principally of cordierite is selected for treatment. A slurry consisting essentially of powdered alumina, powdered Cr O and an aqueous solution of Cu(NO is prepared, and the honeycomb support structure is coated with this slurry by repeated dipping and drying until a loading of 25% by weight based on the weight of the dry, coated structure is obtained. The coated structure is then fired at 600C. for 2 hours in air, The resultant fired coating consists essentially, in mole percent on the oxide basis as calculated from the proportions of the slurry constituents, of about 48.7% CuO, 30.5% Cr O and 20.8% Al O the Cu-to-Cr atom ratio being about 0.8 and the Al-to-Cr atom ratio being about 0.68.

The catalytic activity of the structure prepared as described is measured according to a bench testing procedure wherein a simulated exhaust gas consisting of about 1.0% carbon monoxide, 250 ppm C H 1.25% 0 water vapor and the remainder nitrogen by volume is passed through the structure at a space velocity of about 15,000 hr. while the conversion of carbon monoxide and propylene to carbon dioxide and water is monitored. The extent of conversion of each oxidizable constituent is proportional to the temperature of the reaction environment as well as to the acitivity of the catalyst. Therefore, the temperatures at which 50% of each constituent is converted is deemed to be a useful relative measure of catalytic activity, with lower 50% conversion temperatures indicating a higher level of activity for the conversion and vice versa.

, After 50% conversion temperatures for carbon monoxide and propylene have been established, the structure is fired at 800C. for 24 hours in air to accelerate thermal degradation and then retested. It is then fired at 800C. for an additional 24 hours in air and retested, and finally fired at 900C. for 24 hours in air and retested. The results of this series of tests are set forth in Table l below:

TABLE I 50% Conversion Temperatures (F) Copper Chromite 20% Alumina on Monolith In contrast to the above, a similarly prepared device comprising a honeycomb support structure, an alumina support coating, and an essentially alumina-free copper chromite catalyst, when similarly tested for activity before and after thermal aging at 800C. for 24 hours in air, yields the results shown in TABLE II below. The copper chromite catalyst, at a loading of at least about 20% by weight of the coated structure, consists of about 67 mole percent CuO and 33 mole percent Cr O with a Cu-to-Cr atom ratio of about 1.0. The catalyst is applied from a solution of mixed copper and chromium nitrates and is fired at 600C. for 4 hours to obtain the copper chromite phase.

TABLE ll 50% Conversion Temperatures (F) Pure Copper Chromite on Monolith After After After Oxidizable lnitial 24 Hours- 48 Hours- Constituent Preparation 800C. 800C CO 420 500 550 Propylene 640 700 700 as CuQ o at le s tbtfi 115. -2)/0.81 mole percent of the composition, R being the aluminum-tochromium atom ratio as hereinabove set forth. In P16. 2 of the drawing, which is a ternary diagram of the CuO-Cr O -Al O composition system, these catalysts are encompassed by the triangular region denoted X.

Catalysts within this composition region,'which are characterized by the presence of CuO, CuCr O and CuAl O crystal phases, are resistant to the formation of Cu Cr O and thus provide an excellent combination of activity and thermal stability so as to be quite suitable for use in high temperature oxidation processes such as the treatment of automotive exhaust gases to oxidize carbon monoxide and unburned hydrocarbons present therein. The effectiveness of these catalysts has been demonstrated by the use of automotive engine exhaust systems of catalyst-coated monolithic support structures such as described in Example ll above.

A further understanding of the mode of operation of invention as it is presently understood may be gained by a study of FIG. 3 of the drawing, which is a subsolidus phase diagram of a portion of the copper oxidechromium oxide-aluminum oxide ternary system showing the various crystal phases found in pertinent composition regions of this system after firing for 24 hours at 800C. The portion shown lies on the CuOCr O binary A near the compositions CuCr O and Cu C- r Although the system is presented as being ternary, CuO may reduce to k Cu O, making the system more properly quaternary; however, since the copper contents of the two phases are equivalent, /2 Cu O has been projected onto CuO for convenience in interpreting the diagram.

An understanding of this diagram is facilitated by a consideration of broken line B representing a traverse of the diagram away from the CuO.Cr O binary in the direction of increasing alumina. The starting point T is a copper chromite catalyst with a copper-tochromium atom ratio of about 0.8. While the starting phases after initial reaction of this mixture at some temperature below 800C are undoubtedly CuO and CuCr- O firing for 24 hours at 800C. produces a mixture of CuCr O and Cu Cr O which is substantially less active than pure copper chromite.

As alumina is added to the composition, the regions U and V are reached. in these regions, a CuCr O solid solution (s.s.) phase predominates, although at higher CuO levels some free CuO may also be present. CuCr- O s.s. comprises both CuCr O and alumina, and displays essentially the catalytic activity of CuCr O insofar as the oxidation of CO and hydrocarbons is concerned. However, at these alumina levels, some undesirable Cu Cr O is still present and measurements of the activities of catalysts in these regions indicate that they are still not sufficiently stable for high temperature use.

Referring again to FIG. 3, the further addition of alumina produces a composition falling within region W wherein the presence of Cu Cr O is diminished and CuO and CuCr O s.s. predominate. The combined presence of CuO and CuCr O together with the absence of Cu Cr O, indicates that a composition area demonstrating relatively stable catalystic activity to 800C. has been reached. Further alumina additions produce compositions in regions X and Y which are characterized by the continued absence of Cu Cr O as well as the appearance of a CuAl O s.s. phase consisting of CuAl O and Cr O CuAl O s.s. is not as active as CuCr O for the oxidation of CO and hydrocarbons; however, despite the obvious dilution of highly active CuCr O with moderately active CuAl O the absence of Cu- Cr O should provide a catalyst of improved thermal stability. A study of the activity of compositions in these regions indicates that compositions falling within region X (a CuO-CuCr O s.s. -CuAl O s.s. compatibility triangle) demonstrate the highest level of CO oxidation activity and stability. This region corresponds to the region denoted X on the CuO- Cr O -Al O ternary composition diagram of FIG. 2.

Beyond region Y in the direction of increasing Al O content, A1 0 and Cr O solid solutions are present which have been shown to be of relatively poor catalytic activity and/or stability. These compositions would not be suitable for the oxidation of CO and hydrocarbons at elevated temperatures in automotive emisssions control systems.

The above results indicate that there is a region in the copper chromite-alumina composition system wherein the formation of Cu Cr O is suppressed. If the elimination of Cu Cr O aids thermal stability as has been proposed, then the addition of alumina in quantities which are sufficient to draw the composition out of Cu Cr O containing areas, but which are insufficient to produce catalytically inactive phases, provides a useful means for improving the thermal stability of copper chromite catalysts.

We claim:

1. A leached copper chromite-alumina catalyst having a composition consisting essentially of 525% alumina and the remainder copper chromite by weight, said catalyst consisting essentially of crystal phases of Al O and CuCr O and being resistant to the formation of the crystal phase Cu Cr O said catalyst being formed by:

a. preparing an aqueous solution of a mixture of an aluminum salt or alumina, a copper salt, and a chromium salt or chromium oxide wherein the copper-to-chromium atom ratio exceeds 0.5:l, drying to remove water and sintering to produce a product containing Al O CuCr O and excess CuO; and

b. removing the excess CuO from the product by leaching with a strong mineral acid to produce a catalyst wherein the copper-to-chromium atom ratio is about 05:1.

2. A leached copper chromite-alumina catalyst according to claim 1, wherein the alumina content ranges from about 5 to about 15% by weight. 

1. A LEACHED COPPER CHROMITE-ALUMINA CATALYST HAVING A COMPOSITION CONSISTING ESSENTIALLY OF 5-25% ALUMINA AND THE REMAINDER COPPER CHROMITE BY WEIGHT, SAID CATALYST CONSISTING ESSENTIALLY OF CRYSTAL PHASES OF AL2O3 AND CUCR2O4 AND BEING RESISTANT TO THE FORMATION OF THE CRYSTAL PHASE CU2CR2O4, SAID CATALYST BEING FORMED BY: A. PREPARING AN AQUEOUS SOLUTION OF A MIXTURE OF AN ALUMINUM SALT OR ALUMINA, A COPPER SALT, AND A CHROMIUM SALT OR CHROMIUM OXIDE WHEREIN THE COPPER-TO-CHROMIUM ATOM RATIO EXCEEDS 0.5:1, DRYING TO REMOVE WATER AND SINTERING TO PRODUCE A PRODUCT CONTAINING AL2O3, CUCR2O4 AND EXCESS CUO; AND B. REMOVING THE EXCESS CUO FROM THE PRODUCT BY LEACHING WITH A STRONG MINERAL ACID TO PRODUCE A CATALYST WHEREIN THE COPPER-TO-CHROMIUM ATOM RATIO IS ABOUT 0.5:1
 1. A leached copper chromite-alumina catalyst having a composition consisting essentially of 5-25% alumina and the remainder copper chromite by weight, said catalyst consisting essentially of crystal phases of Al2O3 and CuCr2O4 and being resistant to the formation of the crystal phase Cu2Cr2O4, said catalyst being formed by: a. preparing an aqueous solution of a mixture of an aluminum salt or alumina, a copper salt, and a chromium salt or chromium oxide wherein the copper-to-chromium atom ratio exceeds 0.5:1, drying to remove water and sintering to produce a product containing Al2O3, CuCr2O4 and excess CuO; and b. removing the excess CuO from the product by leaching with a strong mineral acid to produce a catalyst wherein the copper-to-chromium atom ratio is about 0.5:1. 